10 Genes and Alleles p79

Dominant and Recessive Alleles

The factors that control each trait exist in pairs. The female parent contributes one factor, while the male parent contributes the other factor. Finally, one factor in a pair can mask, or hide, the other factor.

Genes and AllelesToday, scientists use the word gene for the factors that control a trait. Alleles (uh leelz) are the different forms of a gene. The gene that controls stem height in peas, for example, has one allele for tall stems and one allele for short stems. Each pea plant inherits two alleles from its parents—one allele from the egg and the other from the sperm. A pea plant may inherit two alleles for tall stems, two alleles for short stems, or one of each.

An organism’s traits are controlled by the alleles it inherits from its parents. Some alleles are dominant, while other alleles are recessive. A dominant allele is one whose trait always shows up in the organism when the allele is present. A recessive allele, on the other hand, is hidden whenever the dominant allele is present. A trait controlled by a recessive allele will only show up if the organism does not have the dominant allele.

A dominant allele is represented by a capital letter. For example, the allele for tall stems is represented by T. A recessive allele is represented by the lowercase version of the letter. So, the allele for short stems would be represented by t.